1. Field of the Invention
This invention generally relates to a storage controller that interfaces between host computer systems and a direct access storage device system. More specifically, the invention relates to deconfiguring cache memory from such a storage controller.
2. Background Art
In a large distributed computer system, a plurality of host computers and devices are typically connected to a number of direct access storage devices (DASDs) comprised of hard disk drives (HDDs). The DASDs may be organized in a redundant array of independent disks, i.e., a RAID array. A RAID array is comprised of multiple, independent disks organized into a large, high-performance logical disk. A controller stripes data across the multiple disks in the array and accesses the disks in parallel to achieve higher data transfer rates. However, utilizing multiple disks in an array increases the risk of failure. The solution in the art is to employ redundancy in the form of error-correcting codes to tolerate disk failures.
Not only is there is a risk associated with the failure of a hard disk drive in a DASD system such as a RAID array, but there is also a risk of failure at a point within a storage controller which controls read and write operations between host computers and the DASDs. The conventional storage controller is typically designed to handle hardware failure. One such storage control designed to handle certain hardware failures is the storage controller utilized in the International Business Machines Corporation's (IBM) Enterprise Storage Server (ESS) system. This storage controller has two storage clusters, each of which provides for selective connection between a host computer and a DASD. Each cluster has a cache and a non volatile storage unit (NVS). The cache buffers frequently used data. When a request is made to write data to a DASD attached to the storage controller, the storage controller may cache the data and delay writing the data to a DASD. Caching data can save time as writing operations involve time consuming mechanical operations. The cache and NVS in each cluster can intercommunicate, allowing for recovery and reconfiguration of the storage controller in the event that one of the memory elements is rendered unavailable. For instance, if one cluster and its cache fails, the NVS in the other cluster maintains a back-up of the cache in the failed cluster.
These dual-cluster storage controllers may also failover to a single node configuration if a memory upgrade is required. During these periods of failover, the system is running on a single node configuration, which is less fault tolerant than the normal dual node configuration.